It is known for a carburetor air-fuel mixture adjustment assembly to include a needle valve body that is threaded into a bore in a carburetor main body. The bore in such an assembly intersects a fuel passage in the carburetor main body. The needle valve body has a shank with a tip, a head and an exteriorly threaded portion between them received in a complementary threaded portion of the bore. The tip of the valve body is positioned in axial alignment with an annular seat or orifice of the fuel passage and can be axially advanced and retracted by rotation of the needle valve body within the receptacle to adjust the air-fuel ratio of a fuel mixture. Axial advancement and retraction of the tip relative to the seat or orifice respectively decreases and increases the cross-sectional area of the flow path through the seat or orifice to decrease and increase the amount of fuel that can flow through the orifice. The needle valve body is rotated by using a tool such as a screwdriver to engage a screw head of the valve body that protrudes from the carburetor main body. In some such assemblies, to prevent inadvertent or uncommanded rotation of the valve body within the bore, a tamper-resistant cap is placed over the screw head and is secured to, or braced against an adjacent structure.
Fuel mixture adjustment assemblies of this type have “slop” or clearance between the respective threaded portions of the needle valve body and the bore which permits some axial and/or radial movement of the tip within the seat or orifice, such as when force is applied to the valve body head or while encountering engine vibration. This axial and/or radial movement can change the shape and size of the effective flow area around the tip enough to result in fuel flow rate changes of up to 20% from an optimum fuel flow rate as determined by the manufacturer. Fuel flow rate changes caused by needle “slop” can result in excessively rich or lean fuel mixtures that undesirably increase exhaust emissions and/or adversely affect engine performance. Therefore, it is desirable to reduce fuel flow fluctuations through the seat or orifice and the resulting affects on exhaust emissions and engine performance by limiting needle slop.
To assist in reducing fuel flow fluctuations, it is known to incorporate a spring between the protruding head of the needle valve body and the main body of the carburetor. This creates an axial preload between the mating threads of the needle valve body and the receptacle, thereby reducing the amount of radial and/or axial deflection of the needle valve body within the receptacle and inhibits unintended rotation of the needle valve body.
Another example of a stabilizing system for an air-fuel mixture adjustment needle valve is disclosed in Japanese Patent Application No. 7-346529 filed 12 Dec. 1995 (Japanese Laid-open Publication No. 9-158783 published 17 Jun. 1997). The Japanese Patent Application discloses a carburetor air-fuel mixture adjustment assembly as described above and including a pressure plate made of an elastic material and overlaid on an outer surface of the carburetor main body. The pressure plate includes an aperture that a threaded protruding portion of the needle valve body must be inserted through during assembly. The presence of the pressure plate limits movement of the needle valve body within the receptacle by holding the needle valve body in a centered position.
The carburetor air-fuel mixture adjustment assembly disclosed in this Japanese Patent Application also includes an annular sealing member coaxially disposed between the shank portion of the needle valve body and the receptacle such that the sealing member is compressed between the receptacle and the shank to prevent air from passing between the receptacle and valve body and leaking into the fuel passage. The sealing member is essentially an elongated tube of constant inner and outer diameter that must be forced over a shank portion of the needle valve body then forced into a section of the receptacle shaped to receive the sealing member during assembly. To produce an effective seal against air leakage into the carburetor, machining tolerances must be tight for inner and outer circumferential surfaces of the sealing member, an outer circumferential surface of the shank portion of the valve body, and an inner circumferential surface of the portion of the receptacle receiving the sealing member.